This invention relates to a control circuit for signal level, and is more particularly suitably applicable to the case wherein a sync tip level and deviation of a luminance signal is automatically controlled.
Conventionally, in a video tape recorder, the luminance signal component of an image signal is subjected to a frequency modulation (hereinafter, referred to as an FM modulation), in accordance with the output characteristics of a reproduction head, and recorded on a magnetic tape.
In this FM modulation system, the sync tip level of a luminance signal is modulated, for example, to 4.2 [MHz] and the white peak level thereof is modulated, for example, to 5.4 [MHz] (FIG. 1).
These sync tip level and white peak level are controlled by manually controlling a variable resistance of a carrier/deviation control circuit 1 shown in FIG. 2 in a manufactuaring line.
The carrier/deviation control circuit 1 is arranged such that the base of a PNP transistor Q1 is connected to the output terminal of an operational amplifier OP1 arranged as a voltage follower; the emitter thereof is connected to the inverted input terminal of the operational amplifier OP1, and the junction P1 at which the emitter is connected to the inverted input terminal is connected to a carrier control pin 1A.
The carrier control pin 1A is connected to a reference voltage supply pin 1B for supplying a second reference voltage V through a variable resistor R1, and a current supplied to an FM modulation circuit 2 through the transistor Q1 is controlled to a current I corresponding to a sync tip level by controlling a resistance between the junction P1 and the reference voltage supply pin 1B.
Further, an AC current i corresponding to a predetermined deviation is controlled by controlling a resistance of a variable resistor R2 connected between a deviation control pin 1C connected through an emphasis circuit 3 and an operational amplifier OP2 arranged as a voltage follower and the carrier control pin 1A.
A half H shift circuit 4 prevents crosstalk in such a manner that a half H shift current I.sub.H superimposed on the current I and AC current i at an 1/2 horizontal frequency is supplied to the FM modulation circuit 2 to provide a difference in level with the sync tip level.
Neverthless, in the carrier/deviation control circuit 1, since the sync tip level and deviation is controlled by manually controlling the variable resistors R1 and R2, a problem arises in that trained workers are needed in a manufacturing line and the requirement for adjusting different controls on to a different type of carier/deviation control circuits is difficult to be met as a commodity cycle is shortened.
To cope with this problem, it is contemplated that an automatic control IC 6 composed of a voltage control amplifier 7, DCL/S circuit 8 and operational amplifier OP3 is additionally provided between the pins 1B and 1C of the carrier/deviation control circuit 1 to control a sync tip level and deviation, as shown in FIG. 3, but in this case the additional automatic control IC 6 is needed and thus a cost is increased, as well as a metal film resistor R3 is necessary between the automatic control IC 6 and the pin 1B.
Thus, as shown in FIG. 4, it is contemplated that this automatic control IC 6 is contained in the carrier/deviation control circuit 1, but in this case, although the additional IC is not necessary, the number of pins is increased from 2 to 4 and an external resistor R4 is necessary between the operational amplifier OP3 and the junction P1 to alleviate an influence due to the temperature characteristic of a resistance.
Furthermore, FIG. 5 shows the prior art wherein the control circuit uses automatic control circuit corresponding to EVR.
A carrier/deviation control circuit 11 controls a gain to an input signal V.sub.in by a DC control signal S1 input to a voltage amplifier 12, which has an output connected to the emitter of a transistor Q11 and the inverted input terminal of an operational amplifier OP11 composed of a voltage follower, through an internal resistor R1.
A first reference voltage (2V) is connected through an internal resistor R2 to the junction P1 at which the emitter of the transistor Q11 is connected to the inverted input terminal and thus a DC current I1 corresponding to a potential difference between a second reference voltage (V) supplied to the noninverted input terminal of the operational amplifier OP11 and the first reference voltage (2V) is supplied to an FM modulation circuit 3 through the transistor Q11.
The sync tip level of a luminance signal is set by the DC current I1.
The carrier/deviation control circuit 11 superimposes an AC current i flowing through the internal resistor R1 in accordance with a potential produced between the output voltage of the voltage control amplifier 12 and the junction P1 on the DC current I1 and supplies the same to the FM modulation circuit 13 to thereby control the sync tip level and deviation of the luminance signal.
Neverthless, this carrier/deviation control circuit 11 has a problem that since a luminance signal Vin is converted once to a change of a voltage by the voltage control amplifier 12 and converted to an AC current i by the internal resistor R1 and then supplied to the FM modulation circuit 13, the voltage control amplifier 12 has a severe dynamic range, and thus an IC is difficult to be disigned only by lateral PNP transistors.
Further, although a conversion to a current is carried out once by a low impedance circuit, when a circuit must be designed only by the lateral PNP transistors, these lateral PNP transistors must be large due to a small current capacity.
Further, since a lateral PNP transistor has a small current amplification h.sub.fe, a deviation of the current amplification h.sub.fe cannot be ignored and there is a possibility that a signal waveform may be distorted due to a phase dislocation occurred when an AC signal i changing in accordance with an image signal V.sub.i flows from an emitter to a collector.